1. Field of the Invention
The present invention relates to a method of manufacturing a multi-layer circuit board that has inner via-hole connection via conductive paste.
2. Description of the Related Art
In recent years, as electronic devices are formed into smaller in size and higher in-density, a multi-layer circuit board has been strongly needed not only for industrial use but also for consumer products.
To provide a multi-layer circuit board with high-density, microscopic and multi-layered circuit patterns have been developing. At the same time, a thin circuit board has also been needed.
Such a multi-layer circuit board requires highly reliable interlayer connections in which circuit patterns formed on the layers are connected via inner via-holes. For example, Japanese Patent Unexamined Publication No. H06-268345 introduces a method of manufacturing a high-density circuit board that offers inner via-hole connections via conductive paste filled in via-holes.
Hereinafter will be described a conventional method of manufacturing a multi-layer circuit board, in particular, a four-layered circuit board.
First, a description will be given on the manufacturing process of a double-sided circuit board that is the base of a four-layered circuit board.
FIGS. 5A through 5F illustrate the manufacturing process of a conventional double-sided circuit board, showing step-by-step sectional views until the circuit board is done.
Prepreg sheet 21 is 250 mm×250 mm square and approx. 150 μm in thickness. Prepreg sheet 21 is formed of composite material, for example, a non-woven fabric made of aromatic polyamide fiber impregnated with thermosetting epoxy resin. Mold releasing film 22, which is made of polyethylene terephthalate or the like, has a thickness of approx. 10 μm. On one surface of film 22, a silicone mold releasing agent is applied. Metal foil sheets 25a and 25b, made of Cu or the like, 12 μm-in thickness, cover each surface of prepreg sheet 21. Through-holes 23 in prepreg sheet 21 are filled with conductive paste 24, through which foil sheet 25a establishes electrical connection to foil sheet 25b. 
As shown in FIGS. 5A and 5B, through-holes 23 are formed at predetermined positions, by laser processing or the like, in prepreg sheet 21 having mold releasing film 22 on both surfaces.
Through-holes 23 are, as shown in FIG. 5C, filled with conductive paste 24. Specifically, prepreg sheet 21 with through-holes 23 is put on a printer table (not shown), and then conductive paste 24 is directly printed on mold releasing film 22. In the printing process, mold releasing film 22, particularly the upper one, not only serves as a printing mask but also keeps the surface of prepreg sheet 21 clean.
After that, mold releasing film 22 is removed from both surfaces of prepreg sheet 21, as shown in FIG. 5D.
FIG. 5E shows a laminated state in which lamination plate 26b, metal foil 25b, prepreg sheet 21, metal foil 25a, and lamination plate 26a are stacked up in the order. The laminated structure undergoes heat pressing. The application of heat and pressure not only compresses down prepreg sheet 21 as shown in FIG. 5F (where, t2 measures approx. 100 μm), but also attaches metal foil sheets 25a, 25b with prepreg sheet 21. In this way, metal foil sheets 25a and 25b have electrical connections via conductive paste 24 filled in through-holes 23 formed at predetermined positions in the prepreg sheet. Following the above-described steps, a circuit pattern is formed (not shown) by selective etching on metal foil sheets 25a and 25b. The double-sided circuit board of FIG. 5F is thus obtained.
Next, a conventional method of manufacturing a multi-layer circuit board is described. Taking a four-layer circuit board as an example, the sectional views in FIGS. 6A through 6D illustrate a method of manufacturing a multi-layer circuit board.
First, as shown in FIG. 6A, double-sided circuit board 30 as a core circuit board, and prepreg sheets 21a, 21b are prepared. Circuit board 30 having circuit patterns 31a and 31b thereon is formed through the processes from FIG. 5A to FIG. 5F. Prepreg sheets 21a and 21b have through-holes filled with conductive paste 24.
The aforementioned materials are stacked up, as shown in FIG. 6B, together with following components in the order: lamination plate 26b, metal foil 25b made of copper or the like, prepreg sheet 21b, double-sided circuit board 30, prepreg sheet 21a, metal foil 25a, lamination plate 26a. The laminated structure is held by buffer material (not shown) and positioned on a heat press plate (not shown) to undergo heat pressing. With the application of heat and pressure, prepreg sheets 21a and 21b are compressed down (t2 measures approx. 100 μm), metal foil sheets 25a and 25b are bonded with double-sided circuit board 30, and circuit patterns 31a and 31b establish inner-via-hole connections to metal foil sheets 25a and 25b via conductive paste 24.
In the heat pressing process, to protect metal foil sheets 25a and 25b from formation of wrinkles, lamination plates 26a and 26b are usually formed of material having a coefficient of linear expansion equivalent to that of the metal foil (for example, 18×10−6/° C. for copper foil).
Etching on selective portions of each surface of metal foil sheets 25a and 25b forms circuit patterns 32a and 32b, respectively. A four-layered circuit board is thus obtained.
To manufacture a circuit board having six or more layers, a core circuit board having four or more layers is needed instead of double-sided circuit board 30. Carrying out the process repeatedly from FIGS. 6A through 6D can produce a multi-layer circuit board having six or more layers.
Prepreg sheet 21 described here has a thickness of 100 μm after being compressed.
As the demand for a thin board grows, a request for a high-quality multi-layer circuit board with slim structure and reliable connection resistance has become stronger.